Discussion in 'The Projects Forum' started by Gorton, Dec 29, 2014.

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Jun 20, 2013
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Firstly to those reading this who helped me with my last LED art project, thanks ever so much!! It looks fantastic and I couldn't have done it without you. If you want the finished result in pictures just say.
I am now about to embark on my next and feel confident going forth thanks to the knowledge you instilled within. However there are a few things I'd like to ask first so as I'm on the straight and narrow so he goes:

Do LED's have to be used for certain voltages? I.e so long as the resistors are in place can you connect it to a massive voltage without it overheating and causing a fire?? (I'm using 24v this time instead of 12)

Using higher voltage has a significant effect on the amount of current to have to use yeah?!

If I'm using higher voltage will I need thicker hook up wire or is that more to do with current carrying ability?? I'm thinking of using 22AWG
This piece will have to be finished in fabric so fire is obviously a concern ... If its gonna cause a fire will it happen within a matter of minutes or could it take hours to get to temperature?

Thanks ever so much in advance
Joe

2. John P AAC Fanatic!

Oct 14, 2008
1,636
225
To take the 2nd question first--no, you're the one who has to make sure that the voltasge has no significant effect on the current! And you do it by selecting a resistor that drops that extra 12V so as to keep the current constant. And as for the 1st question--if something overheats, it may be the resistor. You have to choose it not just to keep the current where you want it, but to handle the power loss without damage.

3. Brevor Active Member

Apr 9, 2011
282
56
Not really, if the LED (s) remain the same you want the current to also remain the same. Going from 12 to 24 Volts most likely you could just double the resistor value. But you need to calculate the power the resistor will dissipate and use a resistor with a wattage rating that is appropriate.
If the current through the LED's is the same the wire size will not have to be changed.

Jun 20, 2013
51
3
Right ... With the calculator I used, I'm guessing the overall current requirements will be less because in doubling the voltage, you can fit more LED's on a string therefore requiring significantly less resistors which brings the current down alot right?

Can you get any devices that regulate the voltage coming from a DC supply like a battery?

Am I right in assuming that current and power are both kind of 'resultants' of how many devices are within a circuit and the only factor that's a constant from start to finish is the voltage?

Thanks guys and gals

5. blocco a spirale AAC Fanatic!

Jun 18, 2008
1,455
369
There are many ways of doing this so it might be easier if you just state how many LEDs you wish to drive, your supply voltage range, and the forward voltage and current of your LEDs.

Jun 20, 2013
51
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LED voltage 2.9
Forward current 20mA
453 in total

Driver:
5A
24V
120W

Individual circuits:
8 in a row connected to a 47 ohm 1/4W resistor.

They are not all linked together, yet and I've just tested each individual circuit with the resistor attached to a wire leading directly to the driver and its overheating massively. Why is this when it should all be electronically sound?
I guess once its all linked together and all 453 are working together the wiring is still going to get hot??
The input is supposed to be 240V 5A input and I've only got a 1A fuse t the moment so I've put that in.

7. Bernard AAC Fanatic!

Aug 7, 2008
4,230
412
I would use 7 LEDs in series with 180 ohm. Total load around 1.3 A @ 24 V

8. MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Something wrong with your math. If the PowerSupply is 24V @ 5A, that would be 120W output. Assuming ~75% efficiency, the Line Current at 240V would be (120/240)/0.75 = 0.66A, so a 1A fuse on the line side is fine...

If a single LED is 2.9V at 0.02A, that is 0.058W or 58mW. 453 of them would be be only 26.2W (not counting the power in the resistors), so your supply is a bit of overkill.

7LEDs per string x 2.9V = 20.3V, leaving 3.7V across the resistor. Each resistor dissipates 3.7*0.02 = 74mW. There will be 453/7 = 65 strings

65 parallel strings @ 20mA per string is 1.3A total current from the power supply, which is 24x1.3= 31.2W total.

Last edited: Jan 9, 2015

Jun 20, 2013
51
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Hmmm the thing is I've already wired up all 455 ready for the resistors to go in :-/ the range is 2.8V - 3V per resistor.
What is the actual reason they are overheating?? Current and power are both functions of the demands the components place on the circuit I.e the more components the more current and therefore more power the circuits draws from the supply?

Jun 20, 2013
51
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Let me put it another way - can I change the resistor to stop it from overheating?

11. djsfantasi AAC Fanatic!

Apr 11, 2010
2,899
870
What wiring is getting hot?

453 LEDs hooked up in seres strings of 8 result in 57 strings. If each string is pulling 20ma, the total current drawn through your supply wires is 1.13 A. This is over the ratings for 22 AWG wire., by .21 A.

Also with 8 in series, you don't have enough excess voltage for the resistor to reliably control the current.

Jun 20, 2013
51
3
It's the actual body of the resistor itself that's getting hot.
Not worry I've spit them into 2 banks of 224 in one the remainder in the other to stop that being a problem
Just been doing some reading ... Would using 51 ohm resistors solve the problem completely or just lessens a little?

13. ebeowulf17 Active Member

Aug 12, 2014
682
79
Any chance you could upload a schematic, a sketch, or photos of your circuit?

Your descriptions are a little vague and leave me wondering about series vs parallel connections, how you're factoring voltage drop into your thinking, and a few other potential issues. Seeing the layout would clarify things a lot!

14. djsfantasi AAC Fanatic!

Apr 11, 2010
2,899
870
Also, you may be able to run the LEDs at lower current. This would mean you could use a larger resistor Lower current would mean less heat.

This would be useful, too, because if the Vf of your LEDs is at the low end, a larger resistor is indicated.

I am also waiting for a schematic or diagram.

Jun 20, 2013
51
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Right guys - I've jumped the gun a little and basically finished the whole project!! It looks fantastic and nothing is getting overheated because I put much stronger resistors on which leads me to believe one thing, the Chinese manufacturers supplied the wrong data. It seems from all the other LED's that there is v little difference between te stats for the 3mm LED and the 5mm LED.
I still have a couple of things I need sorting though:
1 - the blue LED circuit is excessively bright so could I bring it down by using a large 1/2W resistor?? What increment would you use to bring the brightness down I.e. 1k ohm or 100 ohm or something?! Or will it simply either work or not work??
2 - I bought a switch the other day believing it was suitable for all voltages as there was no data for it on the packaging then when I opened it up at home it said 12V 30A ... Is it suitable to use with 24V 3A?? Only Im guessing it's the power it won't be able to handle as opposed to the voltage??

I presume of nothing is overheating then it's safe yeah??

Jun 20, 2013
51
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Thanks guys!! Couldn't have done it without you

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Bernard likes this.

Jun 20, 2013
51
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Anybody :-/

18. crutschow Expert

Mar 14, 2008
13,462
3,353
1. Yes, a (larger value) resistor in series with the blue LED should reduce its brightness. Start with a few hundred ohms and work up from there.
2. The switch will likely work okay on 24V at the lower current.

Jun 20, 2013
51
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Cheers crutschow!!! Much Ppreciated

Jun 20, 2013
51
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Right guys. thr board is ready for fitting however on the voltage guage on my truck it says 27.7v but I've designed my to run at 24v as that's what trucks are supposed to run at ... How can I solve this problem?
Resistor maybe??
Some other type of device I can add?
Don't wanna see all my hard work start blowing up infront of mine eyes!!